The invention relates to magnetic resonance spectroscopy. More particularly, the invention relates to a technique for obtaining spectroscopic data in which a selected portion of a solvent signal is shifted away from the signal of interest by partial modulation of the solvent signal.
Magnetic resonance imaging (MRI) techniques are common in the field of diagnostic medical imaging. The MRI scanners apply a uniform magnetic field upon which various gradient fields have been superimposed. The uniform magnetic field homogenizes the spins of responsive material within the object such that the spins are effectively statistically aligned. An excitation RF pulse is then applied to synchronize the spins of the responsive material by directionally xe2x80x9ctippingxe2x80x9d the spins into a plane transverse to the uniform magnetic field. Upon removal of the excitation RF pulse, the spins realign with the uniform magnetic field and, in the process, emit resonance signals. Differences in these resonance signals attributable to each nuclear species are detected by the imaging system and are processed to produce a magnetic resonance image which is descriptive of the physiological structure of the subject. In the field of medical imaging the responsive material is typically hydrogen and, for simplicity, hydrogen will be discussed as an exemplary responsive material hereinafter. However it should be realized that hydrogen is not the only responsive material and that the following comments apply to other such responsive materials as well.
When hydrogen is a constituent of a molecule, the electron cloud of the molecule affects the magnetic field strength experienced by the hydrogen nuclei. The variation in the effective magnetic field strength predictably results in a small change to the precession frequency, or spin, of the responsive material. This variation in the precession frequency is manifested as a chemical shift which is different for different hydrogen-containing molecules. In medical imaging of a patient, this chemical shift allows different chemicals within the body to be identified and allows the concentration of such chemicals to be determined. A gradient magnetic field applied in addition to the static field will produce a spatially dependent frequency shift to all the chemical spectra, allowing their localization within the field of view. In particular, a Fourier transformation may be employed to calculate a chemical shift spectrum from the resonance signal, decomposing the signal into its frequency and spatial components with each frequency corresponding to a component of a specific chemical at a specific location in space. The spectroscopic and spatial information thereby obtained may be utilized in the fields of magnetic resonance spectroscopy (MRS) or magnetic resonance spectroscopic imaging (MRSI) depending on whether data is obtained in one dimension or more than one dimension, respectively. As compared to MRI techniques, which provide structural information about a subject""s physiology, MRS and MRSI techniques generate a plot representing the chemical composition of the imaged region in conjunction with the structural information, thereby providing information about the chemical functioning of the subject.
However, these spectroscopic imaging techniques utilizing hydrogen nuclei may be problematic when applied to human patients due to the presence of the hydrogen nuclei in highly prevalent water and lipid molecules. In particular, the hydrogen found in water and in lipids can produce very strong resonance signals which can mask the resonance signal of lower concentration compounds of interest, usually metabolites such as choline, lactate, or creatine.
These solvent signals, that is, the water and lipid signals, may be suppressed to better discern the resonance signals of compounds of interest, such as metabolites, in MRS and MRSI. Examples of suppressive techniques include chemical shift selective (CHESS) saturation and short-time inversion recovery (STIR) for water and lipid suppression respectively. CHESS suppression may be used for short echo time chemical shift imaging and allows an operator to select the degree of solvent suppression to be employed. Because the CHESS suppression technique is functionally independent of the imaging process, however, mismatches between the chemical suppression and the image can lead to image artifacts and processing irregularities, including apparent negative concentrations of the solvent, which may impact contiguous frequencies of interest within the image. Indeed, complete water suppression by means of CHESS techniques may create a situation where information which might otherwise be determined from the water signal is unavailable. In particular, some solvent signal may be desired to provide frequency reference information during image reconstruction. It may therefore be desirable to preserve the frequency information or other information associated with the solvent signal.
It is also known in the field of MRS and MRSI that two-dimensional spectral-spatial pulses may be utilized which are selective in space and in frequency. These spectral-spatial pulses synchronize the refocusing pulses with the time-varying magnetic field gradients to provide the desired spatial and frequency selectivity. In general, these spectral-spatial pulses can be designed to avoid the excitation of unwanted chemical species, and may thereby be used to avoid or minimize a resonance signal from water or lipids. The spectral spatial pulses, however, because of their two-dimensional nature, produce long echo times which may be undesirable in many examination contexts.
One technique utilizing spectral-spatial pulses in MRS and MRSI employs two spectral-spatial pulses as the final two pulses of a point resolved spectroscopy (PRESS) sequence, i.e. a 90xc2x0 tip angle RF pulse followed by two 180xc2x0 refocusing pulses. While this technique will suppress the undesired water and lipid signal, components of the undesired water and lipid signal will continue to contaminate the signal of interest, producing erroneous frequency and concentration information. A further technique is to perform two separate acquisitions, one with water suppression and one without. The two separate data sets may then be used to perform artifact removal algorithms, i.e. B0 correction, and water subtraction to enhance the signal of interest, though acquisition time is further increased due to the second acquisition.
Dual-band selective excitation is another technique used in MRS and MRSI. In dual-band selective excitation the water and non-water parts of the sample are differentially excited such that the water is only partially excited relative to the metabolites or other compound of interest. The resulting spectra therefore have a reduced water signal relative to the metabolites. However, even this reduced water signal interferes with the metabolite signal and prevents the application of artifact removing algorithms.
In addition, combined use of dual-band selective excitation and spectral-spatial pulses may allow for full modulation of a solvent signal. However, as noted above, some solvent signal is generally desired to provide frequency reference information during image reconstruction. Ideally, the information provided by the water signal would be available, but would not contaminate the signal of interest. In addition, the two-dimensional nature of the spectral-spatial pulses may lead to longer echo times and, therefore, to more time consuming acquisition sequences than may be feasible. A technique is therefore needed which allows for the rapid partial modulation of a solvent signal from a signal of interest.
The invention provides a method of partially modulating a solvent spectrum from one or more spectra of interest. In accordance with one aspect of the technique, an imaging pulse sequence utilizing a first solvent suppressing pulse and the imaging pulse sequence utilizing a second solvent suppressing pulse are applied to a sample in an alternating fashion. The first solvent suppressing pulse sequence suppresses a solvent spectrum by a first amount and the second solvent suppressing pulse suppresses the solvent spectrum by a different amount.
In accordance with another aspect of the present technique, a method is provided for forming a magnetic resonance spectroscopic image. The method includes iteratively applying an imaging pulse sequence to a subject. The imaging pulse sequence is alternately preceded by a first solvent suppressive pulse and a second solvent suppressive pulse which differ in the amount of solvent suppression they produce. A resonance data set resulting from each imaging pulse sequence is acquired before applying the alternating imaging pulse sequence. An acquisition matrix is formed from the resonance data sets and is processed. The processed matrix is used to construct a spectroscopic image.
In accordance with another aspect of the present technique, a magnetic resonance spectroscopy imaging system is provided. The system includes a magnetic resonance scanner, one or more control and acquisition circuits operably connected to the scanner, system controller circuitry operably connected to the one or more control and acquisition circuits, and an operator interface station operably connected to the system controller circuitry. The system controller circuitry is configured to iteratively apply an imaging pulse sequence to a subject, to acquire a resonance data set resulting from each imaging pulse sequence before applying the alternating imaging pulse sequence, and to process the resonance data sets to produce a spectrum of interest. The imaging pulse sequence is alternately preceded by a first solvent suppressive pulse and a second solvent suppressive pulse which differ in the amount of solvent suppression they produce.
In accordance with another aspect of the present technique, a computer program for partially modulating a solvent spectrum relative to one or more spectra of interest is provided. The computer program includes a machine readable medium for supporting machine readable code and configuration code stored on the machine readable medium. The configuration code contains instructions for applying two or more imaging pulse sequences to a subject. The imaging pulse sequences are alternately preceded by a first solvent suppressive pulse and a second solvent suppressive pulse which differ in the amount of solvent suppression they produce.
In accordance with another aspect of the present technique, a magnetic resonance spectroscopy imaging system is provided. The system includes a magnetic resonance scanner, one or more control and acquisition circuits operably connected to the scanner, system controller circuitry operably connected to the one or more control and acquisition circuits, and an operator interface station operably connected to the system controller circuitry. The system controller circuitry comprises a means for producing a magnetic resonance spectroscopic image in which a solvent spectrum is partially modulated from one or more spectra of interest.